The invention relates to a process for the preparation of isocyanates in a system network comprising an isocyanate production plant, a chlorine production plant and a phosgene production plant, in which carbon dioxide formed as a by-product is partially to completely condensed out with the gaseous chlorine formed in the chlorine production plant and enters into the phosgene preparation process and, after the preparation of phosgene, the predominant part of the carbon dioxide formed is thereby sluiced out of the system network in gaseous form.
In the preparation of isocyanates by phosgenation of the corresponding primary amines, hydrogen chloride gas is formed as a by-product. The hydrogen chloride gas is conventionally contaminated with gaseous substances from the phosgene synthesis or other process stages, such as e.g. carbon oxides (CO and CO2) and phosgene. It can be reused for the preparation of isocyanates by preparing from it chlorine, which is required for the phosgene synthesis.
The preparation of chlorine from hydrogen chloride is adequately known from the prior art. There are various electrolytic processes for the preparation of chlorine which start from hydrochloric acid (HCl (aq.)) or alkali metal chlorides. In addition, direct catalytic oxidation of gaseous hydrogen chloride (HCl (g))4HCl(g)+O2(g)→2Cl2(g)+2H2O(g),called the Deacon process in the following, is increasingly gaining in importance.
Certain purity requirements are to be imposed on the educt gas stream of a Deacon plant. It is thus known from DE 10 2006 024 549 A1 that the CO content in the intake stream should be less than 1 vol. %, in particular less than 0.5 vol. %, based on the total volume of the intake stream. A higher CO content may impair the plant availability as a result of accelerated catalyst deactivation. It is furthermore known also to add water to the intake stream, in addition to the educts hydrogen chloride and oxygen, in order to smooth the temperature distribution within the catalyst layer (JP 2001 019 405 A). The positive influence of water manifests itself inasmuch as it can at least partly compensate the negative influence of CO. To the extent that the rate of the highly exothermic CO oxidation is slowed down, the metering in of water preferably reduces the development of local hotspots in which the progress of a sintering process which is negative for the activity is preferably accelerated and in which the active component of the catalyst (e.g. ruthenium) is preferably driven out by means of heat. The metering in of water furthermore reduces the formation of volatile metal carbonyl and metal chlorocarbonyl compounds from the catalyst employed, formation of which also already promotes an accelerated discharge of catalytically active metal (e.g. ruthenium) at temperatures below 350° C.
In the sense of a closed production network, it is advantageous to couple a plant for production of chlorine from hydrogen chloride with an isocyanate production plant in order thus to minimize the outlay on apparatus, the energy consumption and therefore the costs by the maximum utilization of circulation flows. In this context chlorine production via a Deacon process is particularly advantageous.
In this connection, however, particular impurities which are contained in a hydrogen chloride stream originating from an isocyanate production may be problematic. There may be mentioned here in particular carbon dioxide, which would become ever more concentrated in the circulation streams if no particular measures were taken.
It is therefore conventional to sluice out carbon dioxide via a suitable purge stream (waste gas stream). In a combined Deacon/isocyanate production plant, this is effected according to the prior art such that the chlorine-rich product stream obtained in the Deacon process is introduced, after drying and compression, into a distillation column. In this context, the overhead condenser of the distillation column is used for chlorine condensation and the bottom evaporator is used for driving out CO2, unreacted oxygen and where appropriate other volatile components. This prior art is known, for example, from EP 0 329 385 A2 or DE 10 2006 023 581 A1. The chlorine separated off can now be fed into the isocyanate process and used for phosgene synthesis.
The gas stream driven out in the chlorine distillation contains the unreacted oxygen (oxygen is conventionally employed in a large excess in the Deacon process), the CO2 and further gaseous substances, and, if necessary after a washing stage, is recycled into the Deacon reaction. By the recycling of the residual gas, a circulation is built up and a concentration of gases, such as e.g. CO2, which must be sluiced out, occurs.
For this, some of the oxygen-containing residual gas recycled into the Deacon reaction is sluiced out of the circulation and the concentration of those gaseous components, which do not further react in the following process stages (such as oxygen) in the circulation stream is thus prevented. The sluicing-out stream must be treated before release into the environment, since is still contains residual chlorine.
The prior art for removal of chlorine from waste gases is a washing with sodium hydroxide solution (see e.g. Ullmann's Encyclopedia of Industrial Chemistry 2006, Chlorine chapter, in particular p. 80, Treatment of Gaseous Effluents), in which the chlorine is absorbed in the wash liquid and reacted chemically:Cl2+2NaOH→NaCl+NaOCl+H2O
In order to remove the chlorine reliably, the washing is operated with an NaOH excess. Because of this excess, however, all the CO2 is also removed at the same time:CO2+2NaOH→Na2CO3+H2O
This unselective absorption of chlorine therefore has the disadvantage when used in a combined Deacon/isocyanate production plant that in this sodium hydroxide solution is consumed not only for absorption of chlorine, but also for absorption of CO2, which is in itself unnecessary. A further disadvantage is that Na2CO3 may precipitate out in the wash liquid and block the washer. In such a case the functional capacity thereof no longer exists and the Deacon process must be closed down.
A selective absorption of chlorine from a CO2-containing gas stream, however, has also already been dealt with in several publications. Thus, for example, US H1417 describes a process in which chlorine in absorbed in a wash column with a mixture of sodium thiosulfate (Na2S2O3) and sodium hydroxide solution and is reduced. In this context, sodium hydroxide solution is metered in until a pH of approx. 8.5 is established in the wash liquid. At this pH, no NaOH but NaHCO3 is present. This is due to the fact that so little NaOH is fed in that only a part of the CO2 is absorbed, NaHCO3 is formed and this is then consumed in the reduction of the chlorine and the CO2 is liberated again:10CO2+10NaOH→10NaHCO3 4Cl2+Na2S2O3+10NaHCO3→8NaCl+2Na2SO4+5H2O+10CO2 
A disadvantage of this procedure is the danger of decomposition of the sodium thiosulfate, during which sulfur precipitates may block the wash column. This decomposition occurs if e.g. the pH of the absorption solution becomes too low locally in the column. For this reason, inter alia, this process is not used on waste gas streams with a relatively high chlorine content and flow rate.
Sulfite is stated as an alternative reducing agent in US H1417, but it is pointed out that sulfite tends to form SO2 particularly readily in the event of variations in the pH. This SO2 would contaminate the gas stream to be purified.
In addition, the use of a reducing agent, whether thiosulfate or sulfite, is expensive.
DE 24 13 358 A1 proposes a multi-stage absorption of chlorine with sodium hydroxide solution, fresh sodium hydroxide solution being led in counter-current to the chlorine- and CO2-containing gas stream. The sodium hydroxide solution is metered in such that it reacts completely with the CO2 to give NaHCO3, which then reacts again with chlorine and thereby liberates the previously bonded CO2:CO2+2NaOH→2NaHCO3 Cl2+2NaHCO3→NaCl+NaOCl+H2O+2CO2 
A disadvantage of this process is the difficult process procedure refereed to in DE 24 13 358 A1 for complete absorption of chlorine. There is furthermore the danger at a low pH that the hypochlorite reacts further to give the chlorate. Chlorate can be decomposed only with great difficulty, compared with hypochlorite, so that the waste water formed presents a disposal problem. Since also no alkali excess is present, the hypochlorite solution formed is unstable and can easily decompose.
WO 2004/037 718 A2 also describes the removal of substances such as phosgene, solvent residue, CO2 etc. from the hydrogen chloride gas to be employed by absorption in water. The hydrochloric acid formed by this procedure must then be dissociated into hydrogen chloride gas and water in an involved manner, which causes considerable costs.
Summarizing, it therefore remains to be said that the unselective absorption of the chlorine from the CO2-containing purge stream to be sluiced out results in an increased demand for sodium hydroxide solution, while the methods known from the prior art for selective absorption of chlorine from a CO2-containing purge stream have a number of other disadvantages. There was therefore a need for a possibility for sluicing out a gaseous, CO2-containing stream from a combined Deacon/isocyanate production plant without the disadvantages described above thereby occurring.